Radio amplifiers are intended to amplify radio signals without adding distortion. With power amplifiers in particular, it is important that this is also done with good power efficiency. All analog amplifiers have limitations with respect to output power, and the power efficiency of amplifiers generally is better when the amplifiers are operated so that their output signals are close to the amplifier's maximum output power. However, this is also the operating region where nonlinearities are manifest, introducing unwanted spurious responses in the amplifiers' output signals.
These nonlinearities are manageable for simple, constant-envelope signals, like those used in the conventional GSM system. However, the modulated baseband signals used in the Long-Term Evolution (LTE) system developed by members of the 3rd-Generation Partnership Project (3GPP) and the composite signals used in systems using aggregated carriers typically have very large peak-to-average ratios. Accordingly, it is difficult to develop power amplifiers that can operate with high efficiencies with these signals, while still introducing low enough levels of non-linear distortion.
To mitigate this problem, techniques referred to as crest factor reduction (CFR) methods are often used to reduce the signal dynamics of the baseband signal. With these approaches generally, the input signal to a power amplifier is modified, or “pre-processed,” so as to reduce the peak-to-average ratio of the signal, while avoiding the introduction of excessive distortion into the signal via the pre-processing.
Traditional solutions use an iterative approach to this pre-processing, combining amplitude limiters and in-band filtering. One solution is to hard-limit the signal, to remove the peaks from the signal, followed by in-band filtering of the hard-limited signal. Typically, multiple iterations are used to get good performance. This approach, however, creates very tough requirements on the filters used for the in-band filtering.
Another alternative is to calculate a signal that represents that part of the input signal where the signal amplitude is above a threshold amplitude value. This calculated signal is then filtered, with an in-band filter, and then this filtered signal is subtracted from the original input signal. This approach imposes less stringent requirements on the filters used to provide the in-band filtering. This method and related methods can work well if several iterations are performed, but the cost of these solutions, in terms of component size and/or current consumption, can be high, which makes these techniques unsuitable for small-cell solutions, such as pico cells, femto cells, home base stations, etc., where unit size and power consumption are especially important issues.
Still another approach is to identify areas of the signal where the signal is above a threshold value, followed by applying a smoothed reduction in gain for the signal, in the vicinity of each overshoot, such that the signal peaks fall below the threshold.
Common to all of these conventional methods is that they aim to limit the amplitude of the signal in such a way that the signal's peak amplitude remains below a certain threshold. To do this, the original signal has to be distorted. In other words, the compensated signal, with reduced peaks, can be viewed as the original signal added to a distortion signal. Strict requirements apply to both out-of-band characteristics of this distortion, so as to avoid the transmitting of excessive distortion on out-of-band frequencies, and in-band distortion, so as to avoid having in-band distortion that disturbs the information signal to an undesirable extent. For all solutions, there is a careful balance between in-band and out-of-band distortion.
This crest factor reduction problem, however, becomes more difficult to solve when the aggregated signal consists of non-contiguous carriers. Conventional techniques can be used to perform the crest factor reduction with adequate quality in these cases, but these methods are typically implemented in situations where calculation efforts, power consumption, and implementation cost are of relatively minor importance, e.g., in large and powerful radio base stations. Radio communication systems are more and more working with high-speed/small-cell scenarios, however. For these scenarios it is important to have methods that perform adequate crest factor reduction with minimal impacts to power consumption and implementation cost. These same considerations apply to crest factor reduction implementations in mobile units, as well as in small-cell radio base stations.